Automatic transmissions to be mounted on vehicles such as automobiles include planetary gear sets and a plurality of frictional engagement elements such as multiplate clutches and multiplate brakes. The plurality of frictional engagement elements are selectively engaged depending on an operation state of an engine so that gears are automatically shifted to predetermined gear positions. The frictional engagement element includes a plurality of friction plates arranged with clearances therebetween, and a piston configured to press the friction plates. The piston moves between an engaging position where the friction plates are brought into an engaged state by pressing the friction plates, and a releasing position where the friction plates are brought into a released state by releasing the pressing.
Japanese Unexamined Patent Publication No. 2001-65607 discloses a frictional engagement element to which a spring member configured to urge the piston toward the engaging position is applied. In this frictional engagement element, the friction plates are pressed by the piston subjected to an urging force of the spring member, and hence the friction plates are engaged with each other so as to have a predetermined engaging force. Note that, when a great engaging force is required, the piston is assisted by urging the piston with a hydraulic pressure.
In the frictional engagement element of Japanese Unexamined Patent Publication No. 2001-65607, the engaged state is produced solely by the urging force of the spring member. Thus, when the frictional engagement element is switched from the released state to the engaged state, there is a risk of delay in the switching of the state. In order to address the switching delay, it is conceived to move the piston at high speed by increasing the urging force of the spring member. In this case, however, there is a risk of an engagement shock along with sudden engagement of the friction plates.